In many steps of electronic device fabrication, it is important to measure surface shapes or surface contours with typical peak-to-valley dimension of 10 to 100 micrometers (.mu.m). This range of dimensions is typically too large to be measured by optical interference techniques. Therefore, one usually uses for such dimensions a depth gauge with a stylus which touches the surface of the sample. The position of the stylus in the direction perpendicular to the sample surface can be sensed by conventional electronic means. The most widely used mechanisms are ones that utilize a linearly variable differential transducer (LVDT), such as those used in the depth gauges with the trade names Dektak and Talysurf. Several disadvantages of these instruments include the following: (1) the stylus can damage the surface of the sample if the sample is soft or brittle; (2) the mechanical stylus provided with such instruments necessarily has a width which prevents it from reaching into narrow holes and grooves of the surface; (3) this type of measurement is inherently slow. These three impediments, at least in the use of LVDT instruments, become more significantly a problem when one looks at the particular problem occurring in silicon power transistor manufacturing, as will be explained in more detail hereinafter.
The fabrication of semiconductor devices, such as power transistors and thyristors, involves the etching of deep grooves (e.g., 30-100 .mu.m) into the surface of, for example, silicon wafers. These grooves electrically separate the individual devices from one another. Moreover, these grooves are the site where the high collector field of the transistor meets the surface and where the passivating glass and oxide layers have to be applied. Therefore, the groove depth has to be monitored and controlled in manufacturing the device to tolerances of about .+-.5%. Since the dimensions are large compared to the wavelength of light and since the surfaces can be rough in the form of etched pits, the usual optical interference techniques can not be used. Furthermore, the light section microscope technique used heretofore is relatively slow and somewhat inaccurate due to being dependent upon the operators' skill. See p. 61 of The Microscope, Vol. 21, First Qt. 1973, pp. 59-66 for a description by H. E. Heller of the light section technique.
Reference is also made to two articles: "Electro-optical Techniques for Measurement and Inspection" by D. P. Bortfeld et al. published in the RCA Engineer 26-2 September/October 1960, pp. 75-80, particularly FIGS. 7 and 8, and "Optical Profilometer" by Y. Fainman et al. Applied Optics, Vol. 21, No. 17, 1 Sept. 1982, pp. 3200-3208 for descriptions of a profilometer.
The conventional profilometer can not measure the contour of steep regions since the reflected light beams are masked from the optics. There is a need, therefore, in the art for a profilometer capable of measuring steep contoured surfaces. The relative steepness of a surface is defined herein by the term tilt which is the deviation in surface angle of the region of a surface contour being examined from a reference plane suitably the horizontal plane that is normal (i.e., perpendicular) to the optical axis.